The Institute of Public Policy Research (IPPR) used data from the United Nations to forecast that there would be 9.1 million migrants from abroad by 2030 compared with 5.4 million today. The increase of 3.7 million is the same as adding a city the size of Coventry to Britain's population every two years.

Last year, the Office for National Statistics projected that there would be more than 70 million people living in Britain by 2031 but failed to say how many would be immigrants.

The forecasts were presented to a conference organised by the Local Government Association to discuss how councils could deal with the impacts on their areas of the increase in immigration.

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Jill Rutter, a researcher, said the extra numbers would inevitably create "pinch points" at local level. She said: "We need to acknowledge that migration is here to stay. Our future population will be more diverse ethnically.

"The absolute scale of migration does put pressure on particular public services but the issue is to make sure that the funding of public services is flexible and responsive enough to deal with these localised pinch points."

Prof David Shepherd, from the IPPR, suggested that the Government would have to allow hundreds of thousands of illegal immigrants and asylum seekers to stay in Britain permanently if ministers were going to establish a system of managed migration.

Controversy rages about the 450,000 asylum claims uncovered by the Home Office in 2006 following the foreign prisoner scandal. They were left lying around in boxes, with some files dating from the mid-90s.

Last month, officials admitted that the first 19,000 had already been granted leave to remain in Britain under what the Conservatives claimed was a "stealth amnesty". If cases continued to be granted at the same rate, 165,000 would be allowed to stay.

Liam Byrne, the immigration minister, told the conference that the Government was considering action to deal with the backlog.

Official figures were also thought to underestimate the scale of illegal migrants in the black economy. Figures released by the Government last year showed that twice as many National Insurance numbers were issued to non-Europeans as received work permits.